The present invention relates to digital communications and, in particular, to the tracking of channel response in digital wireless mobile radio systems.
The radio channel in mobile wireless communications may be one of the most harsh mediums to operate. The transmitted signals are often reflected, scattered, diffracted, delayed and attenuated by the surrounding environment. Moreover, the environment through which the signal passes from the transmitter to the receiver is not stationary due to the mobility of the user and surrounding objects. Characteristics of the channel environment also differ from one area to another. Radio propagation in such environments is characterized by multi-path fading, shadowing, and path loss. Among those, multi-path fading may be the most important. Multi-path fading may be characterized by envelope fading, Doppler spread and time-delay spread.
Multi-path waves are combined at the receiver antenna to give a resultant signal which can vary widely in amplitude and phase. Therefore, signal strength may fluctuate rapidly over a small distance traveled or time interval, causing envelope fading. Rayleigh distribution is commonly used to describe the statistical time varying nature of the received envelope of a flat fading signal, or the envelope of an individual multi-path component. In satellite mobile radio and in micro-cellular radio, in addition to the many multi-path waves, a dominant signal, which may be a line-of-sight (LOS) signal, arrives at the receiver and gives rise to a Ricean distributed signal envelope. This dominant path significantly decreases the depth of fading depending on the Ricean parameter, K, which is defined as the ratio of the power in the dominant path to the power in the scattered paths.
Doppler shift is the frequency shift experienced by the radio signal when a wireless receiver, such as a wireless mobile terminal, is in motion. Doppler spread is a measure of the spectral broadening caused by the time rate of change of the mobile radio channel. Doppler spread may lead to frequency dispersion. The Doppler spread in the frequency domain is closely related to the rate of change in the observed signal. Hence, the adaptation time of the processes which are used in the receivers to track the channel variations should be faster than the rate of change of the channel to be able to accurately track the fluctuations in the received signal.
Each of these characteristics of the radio channel present difficulties in tracking the channel to allow for decoding of information contained in the received signal. Often, in wireless mobile radio systems, known data sequences are inserted periodically into the transmitted information sequences. Such data sequences are commonly called synchronizing sequences or training sequences and are typically provided at the beginning of a frame of data. Channel estimation may be carried out using the synchronizing sequences and other known parameters to estimate the impact the channel has on the transmitted signal. After determining the channel response, the channel estimator enters a xe2x80x9cdecision directedxe2x80x9d mode where the symbol estimates are used to estimate the channel.
For systems where fading changes very slowly, generally, least square estimation may be an efficient way of estimating the channel impulse response in the presence of additive white Gaussian noise. Because the fading rate is slow compared to the frame rate, the channel estimates can be updated frame by frame. However, for many wireless mobile radio systems, the channel impulse response changes very rapidly over a small travel distance or time interval. With the trend in wireless communications being to move to higher frequency bands, such as in the Personal Communication Systems (PCS), the Doppler spread, hence, the rate of change in the observed signal may be further increased. Even during the reception of the synchronizing sequences, the mobile radio channel response may not be constant. Therefore, the ability to track the channel parameters for fast time-varying systems provides more robust receiver structures and enhances the receiver performance.
The most commonly used channel tracking methods are the Least Mean Square (LMS) and Recursive Least Square (RLS) based algorithms. See for example, xe2x80x9cOptimal Tracking of Time-varying Channels: A Frequency Domain Approach for known and new algorithms,xe2x80x9d IEEE transactions on selected areas in communications, Vol. 13, NO. 1, January 1995, Jingdong Lin, John G. Proakis, Fuyun Ling. By incorporating prior knowledge about the channel coefficient in the estimation, stochastic based methods have recently been introduced. In contrast to the LMS and RLS, these methods provide for the extrapolation of the channel coefficients in time. More details on these approaches can be obtained in, xe2x80x9cA wiener filtering approach to the design of tracking algorithmsxe2x80x9d, Uppsala University Department of technology and signal processing group, Lars Lindbom, 1995.
One difficulty with the adaptive channel tracker methods is that during the decision directed mode the estimated symbols are used for the channel response adaptation. Therefore, the effect of using potentially incorrect decisions needs to be considered for parameter selection. Tuning of design parameters may result in a trade-off between tracking capability and sensitivity to noise. For example, if the adaptation gain of the channel tracker is very large, then, the channel tracker may become very sensitive to noise and to incorrect symbol decisions. On the other hand, if the adaptation gain is chosen to have a small magnitude, the ability to track the variation of the channel parameters may be lost. Specifically, in those systems where coherent modulation and coherent demodulation schemes are used, these issues become more serious compared to systems where differential modulations are implemented.
In coherent modulation schemes (like coherent Quadrature Phase Shift Keying (QPSK)), even if the channel tracker tracks the magnitude of the channel response well, the channel phase may frequently slip (i.e., the tracker can lock on a wrong phase offset) during a deep fade of the in-phase and/or quadrature phase component of the channel, and the phase would be off by k2xcfx80/m. In other words, the tracker actually tracks well but with an offset, which consequently causes symbol rotation and error propagation. Because the channel phase rotation and symbol rotation are in the opposite direction, a conventional tracker is not able to correct the problem. Thus, all the remaining information symbols may be lost because of this phase rotation until a new frame and synchronization sequence is received.
In light of the above discussion, it is an object of the present invention to provide channel tracking which compensates for variations in the channel including channel fade.
These and other objects of the present invention are provided by utilizing pilots in an information sequence to periodically retrain a channel estimator. Thus, a channel tracker may be synchronized using a synchronization sequence and then periodically retrained using known pilot symbols. Furthermore, the utilization of pilots may allow for the detection of errors in previous channel estimates. When errors are detected, a new channel estimate may be used based on the retraining using the pilot symbols and, optionally, previous errors in symbol estimation may be corrected. Thus, by retraining based on pilot symbols, the propagation of errors may be reduced.
In a particular embodiment, methods and systems are provided which track the channel impulse response of a signal received by a wireless device by estimating the channel impulse response of the received signal during a synchronizing period of a received frame and retraining the channel impulse response during a pilot period within the received frame. Furthermore, the channel impulse response may also be estimated based on estimated symbol values during information periods of the received frame.
In a particular embodiment, the estimate of the channel impulse response is determined by a channel tracker. In such an embodiment, the channel tracker may be placed in a training mode during at least one pilot period of the received frame. Furthermore, the channel tracker may be placed in decision directed mode during at least one information period the received frame. Such a two mode operation may also allow for increasing the gain of the channel tracker when the channel tracker is placed in training mode and decreasing the gain of the channel tracker when the channel tracker is placed in decision directed mode.
In another embodiment of the present invention the channel impulse response of the received signal is estimated during the synchronizing period of the received frame by first determining an average channel impulse response estimate based on a plurality of symbols in during the synchronizing period. The estimated channel impulse responses for symbols in the plurality of symbols, in symbol order, and wherein the initial symbol estimate for a symbol in the plurality of symbols is based on the determined average channel impulse response.
In yet another embodiment of the present invention, phase slip occurrences or channel estimate deviations from the real channel are detected in the signal received by the wireless device. Such a determination may be made by comparing a determined channel impulse response determined during an information period of the received frame to a determined channel impulse response determined during a pilot period of a frame to determine the difference in phase between the channel impulse responses. A phase slip occurrence or channel estimate deviation may then be detected based on the determined difference.
Furthermore, the comparison may be made by comparing a determined channel impulse response determined during an information period of the received frame corresponding to a first information symbol to a determined channel impulse response determined during a pilot period of a frame corresponding to a pilot symbol immediately subsequent to the first information symbol to determine the difference in phase between the channel impulse responses. In such a case, the determined channel impulse response determined during a pilot period of a frame corresponding to a pilot symbol immediately subsequent to the first information symbol may be determined by re-determining channel impulse responses in a reverse direction from determined channel impulse responses corresponding to pilot symbols subsequent to the pilot symbol immediately subsequent to the first information symbol.
In yet another embodiment of the present invention, the determined channel impulse response corresponding to the first information symbol may be discarded and subsequent tracking performed utilizing the determined channel estimate corresponding to a pilot symbol subsequent to the first information symbol.
In a symbol correcting embodiment of the present invention, the determined channel impulse responses for symbols prior to the pilot symbol subsequent to the first information symbol are compared. Backwards re-tracking to prior symbols from the pilot symbol immediately subsequent to the first information symbol is carried out until the difference between the determined channel impulse response corresponding to the pilot symbol subsequent to the first information symbol and a determined channel impulse response corresponding to a symbol prior to the first information symbol is less than a predefined threshold so as to determine an initial symbol. Symbol estimates may be revised utilizing the determined channel impulse response corresponding to the pilot symbol subsequent to the initial symbol and until and including the first information symbol.
As will be appreciated by those of skill in the art, the present invention may also be embodied in a radiotelephone.